So, theoretically, if one were to want to put a R180 diff into a Subaru of the GC persuasion, how much wailing and gnashing of teeth would be involved? Are there ways to play Tetris with the Legos and make it plug and play or would one have to get fabby?

The R180 has different splines than R160 hubs and to make it worse, a different number of cv balls. Driveshaft Shop has special axles to make it work, someone ran off a bunch of adapter spiders to make the CV work, or you can swap the hubs and axles to R180 ones.

By the whole back I mean the subframe, axles, diff, uprights, etc. In others words splines wouldn't matter anymore. Know what im saying?

And pluleeeaase, different spline counts can easily be corrected by the questionable use of a massive hydraulic press to make previously incompatible parts, one single assembly! Less parts=simpler=better!

I found the thread with the guy making the adapter CV parts and flipped through it a little. An interesting solution, and would be great if the WRX axles had the same spline count as the 2.5RS. (Apparently it's 22 spline vs. 24, for the axle sticks) That's an unknown right now.

That to me would seem like the most sanitary solution.

A complete R180 rear suspension from a japanese STI would seem to be THE easy button, but then you would be working with consumables not available in the US.

My gut feeling is the best way is to splice two axle sticks together to the correct length and send them out to have new sticks made to that spec... that way ALL consumables are off the shelf. I'm surprised nobody is doing that.

Subaru being Subaru and never redesigning anything when they don't have to, I'm almost certain the GC STI R180 specific parts are the same as the GD STI R180 specific parts. So you can just use the GD STI parts instead of hunting down JDM only stuff. The only thing though is that you then have to use the special fancy schmancy Group N style brake rotors to fit over the larger diameter parking brake the STIs have (which are probably the same as the GC STI rear rotors) Or there's the GD STI Brembos. I suspect that is why hybrid axles aren't more popular, honestly.

Similarly, given Subaru's laziness, I would be incredibly surprised if the axles have a different spline count between the RS and the WRX.

Maybe your diff had been used, abused, and not properly maintained? Surely that couldn't have happened to a 20 year old car!

I'm not saying don't do it, I'm just saying "stop and think". It'll take you 10 minutes and practically zero $$ to put another R160 in there. Its not your Ej25 that wrecked it. I rally-x'd a 2.5RS a ton, set some FTDs, and never had issues. Lots of PGT wrx's doing stage rally survived just fine if they didn't get taken out by rocks.

There's no reason to install an R180 diff if you aren't also going to use the R180 axles and hubs. The axles and wheel bearings are the weak link regardless. Adapting R160 axles to an R180 just means you'll be breaking axles.